US Open suits bigger hitting. The balls are lighter and less friendly to spin. It is also truer bouncing. Whether it is faster, I don't know. But it is definitely more enjoyable to watch compared to Wimbledon.

There is probably that official court speed rating somewhere but Wimbledon has more aces and TB sets than the US Open and a higher ratio of forced to unforced errors, which points towards faster court speed I guess. Also the bounce could be factored in.http://www.physics.usyd.edu.au/~cross/GrandSlamStatistics.htm
Usually seems like the rallies are a lot shorter at Wimbledon too.